In Christopher Nolan’s 2014 movie Interstellar, Anne Hathaway, starring as Brand, a member of a team of scientist-astronauts sent through a wormhole in search of an Earth-like planet in the face of desperate conditions on Earth, states:
“Time is relative, okay? It can stretch, and it can squeeze, but… it can’t run backwards. Just can’t.”
The movie was celebrated by many critics (myself very much included) and scientists for its realistic engagement with physics, especially general relativity, and for pushing the limits of knowledge to the edge of a black hole. Nolan’s 2020 film Tenet envisions the opposite position from Hathaway’s Brand: it centers on a future technology, an algorithm (sealed by its creator in a physical “black box”) capable of reversing the flow of time, so to speak.
I say “so to speak” because, technically, in the universe depicted in Nolan’s Tenet, time does not exist. Rather, the device known as “the algorithm” is capable only of creating a localized reversal of an object’s entropy, apparently including a person’s, creating the appearance of these objects moving backward through time as seen from the perspective of a forward-moving (non-inverted) observer. An inverted person, however, still has free will and can alter past events or force them into a different trajectory, subject to the limitation that direct physical contact between them and their past self will cause the instantaneous annihilation of both.
In contrast with Interstellar, Tenet embraces the position on time detailed in the physicist Carlo Rovelli’s 2014 book Reality Is Not What It Seems: time isn’t a physical real. Per Rovelli, the universe is composed only of quantum fields, space itself is quantized and has a minimum unit (a tetrahedron at the Planck scale), and time is not necessary in writing the fundamental equations of quantum gravity. The apparent “arrow of time,” as Rovelli noted in his 2017 book The Order of Time, results from the universal tendency of entropy to increase. [i] A full treatment of this approach to quantum gravity can be found in the textbook Rovelli co-authored with the physicist Francesca Vidotto, Covariant Loop Quantum Gravity (2014). That theory, which suggests a “big bounce” cosmology, provides the machinery on which Tenet runs.
Like Interstellar, Tenet is a work of science fiction. It asks that we take on faith that there is a technology—the film’s turnstiles—capable of locally reversing entropy and, hence, the perceived “arrow of time.” If we take Rovelli’s vision of the world as a premise, then one of the scenes in Tenet that might seem the most strained—an event that might seem to transpire solely for the sake of narrative convenience—is entirely realistic. Assuming localized entropy reversal is possible, a fire rushing toward an “inverted” object would cause the object to freeze. It’s fair to criticize Nolan for throwing this information at the audience rapidly, putting it in the mouth of the solider who readies the Protagonist (John David Washington) for his first inversion. My point is simply that, from the standpoint of physics, if we accept the movie’s sci-fi premises, this apparently all-too-clever scene is plausible. As Rovelli explains in Reality Is Not What It Seems:
“If you think about it, all phenomena where we detect the passage of time are co-involved with temperature. The salient characteristic of time is that it moves forward and not backward, that is to say, there are irreversible phenomena. “Mechanical” phenomena—ones that don’t involve heat—are reversible. If we film them, and then run the film backward, we see something realistic. If we film a swinging pendulum, or a stone thrown upward then falling, and then watch the film in reverse, we still see a plausible pendulum swinging, or a stone rising and dropping to the ground.
When the stone reaches the ground, it stops, you might object: if you watch the film reversed, you see a stone leaping up from the ground by itself, and this is implausible. But when the stone reaches the ground and stops, where does its energy go? It heats the ground! At the precise moment when heat is produced, the process is irreversible: the past differs from the future. It is always heat and only heat that distinguishes the past from the future.” 
Sator (played by the inimitable Kenneth Branagh), not knowing that the Protagonist is inverted, drives away from the burning car thinking he has won, and then the effects of the entropy-reversal hit, turning fire into ice. It’s a bit of a Bond Villain moment for Sator, but the physics make sense. On Rovelli’s account of quantum gravity, not only is this moment plausible, but it also depicts how the technology would have to work for the movie’s physics to be coherent.
So, what does it all mean? Let me answer by returning to Interstellar. Critics of the film, like The Atlantic’s Noah Gittell, saw the movie as a statement of Nolan’s position on climate change—and a disappointing statement, at that. At the time, I argued that, to the extent Interstellar was political, it was better understood as a statement on space policy, and that it was not intended by its writers (Christopher and Jonathan Nolan, in consultation with the physicist Kip Thorne) as a depiction of the consequences of gradual, anthropogenic climate change. Instead, it dealt with sudden disaster.
The honor of “being Nolan’s statement on climate change” belongs to Tenet. What motivates the antagonists—an entirely unseen group of future humans who send Sator inverted materials—is a desire to erase the damage done to Earth by past humans. In a resource-barren future world, the only option for survival becomes going back in time to undo the damage that already has been done, even if it means eradicating past humans (cf. the Terminator franchise). It’s hard to imagine a purer representation of the essential crisis of regret humanity faces in dealing with the consequences of an increasingly inhospitable planet, made so by our own hands. In the face of all-but-certain doom, Tenet postulates two potential ethical responses: the Protagonist’s and Sator’s, though Nolan’s Dickensian naming conventions (Sator ≈ Sadist) make it easy to tell where his moral sympathies lie. We can work against the universe’s naturally increasing entropy (as the Protagonist does) or we can accept our fates and become absorbed in our selfishness, try to live lavishly and prepare for the Socratic party at the end of our lives (as Sator prefers). With Tenet, Nolan implies that, if World War III ever comes, it will be fought over the consequences of anthropogenic climate change.
As with the rest of Nolan’s filmography, however, the “surprise factor” in Tenet is driven not by the movie’s political valences, but by its emotional content. Where Interstellar offered a shockingly heartrending look at parental love, and where Inception grappled with the depths of grief, Tenet dares to engage with intimate partner abuse and violence. The “surprise reveal” is that Kat (Elizabeth Debicki) is the movie’s hero, in the sense that it is she who kills the villain, like Anne Hathaway’s Selina Kyle in The Dark Knight Rises. Most of the movie’s drama hinges on the submerged erotic tension between Kat and the Protagonist, which Debicki described in an interview as “a very pure kind of love.” On an emotional level, Tenet is the story of an abused woman who conspires with a time-traveler, of sorts, to save the world. Doing this just so happens to involve killing her abuser, whose rage and violence betray an infuriating impotence—Branagh is genuinely disturbing in the role—in the process. Nolan sets the stakes so high that killing Sator is the obvious moral choice for Kat, but the movie’s satisfaction comes, ultimately, in her act of vengeance. Tenet’s plot works overtime to show that Kat’s actions are fully justified, but their meaning is a rejection of the ideology of abuse that Sator represents.
Abuse, in other words, has an ideological component. Sator’s disgusting anger, pettiness, and brutality make him thoroughly contemptible as a villain, but what Kat acts against are the ideas he stands for: “only I matter;” “if I die, the universe might as well be destroyed;” “I am Godlike;” “I deserve unlimited power, respect, and authority;” “others’ lives are mine to control through violence.”.
It’s precisely this self-serving ideology, which may equally well be termed a psychopathology, that underlies what the historian Christopher Lasch called our “culture of narcissism.” And it’s our narcissistic culture that has feuled the climate catastrophe we now confront. In Tenet, then, Nolan’s message on climate change is clear: if we don’t act to, for instance, replace fossil fuels with renewable energy sources like solar and wind (if we don’t take much bolder action than re-entering the Paris Climate Agreement of 2016, for instance), we’ll find ourselves wishing we could erase the past. If we fail, it will be a result of blind sadistic selfishness.
In reality, we live in a universe without Tenet’s algorithm, one in which entropy invariably increases. (In fact, Rovelli’s work shows, this increase in entropy is what the forward movement of time consists in.) We have only the algorithms of an increasingly invasive surveillance system to hang out with. Ultimately, humanity must either confront the reality of gradually impending doom, encroaching on our descendants if not on us, the doom whose approach is being sped up by a narcissistic culture, or perish. Only a sadist—a psychological type who, as the literary critic Romana Byrne noted in her 2013 book Aesthetic Sexuality, derives edgy pleasure from combining the grotesque and violent with the beautiful—could accept such a fate. Kat works with others to save the world, but she acts alone to vanquish the film’s embodiment of the ideology that would destroy it in the first place. Critics, such as The Atlantic’s David Sims, who dismiss Tenet as emotionally cold are missing the searing anger Debicki brings to the moment.
[i] See also Michael Marder’s Energy Dreams: Of Actuality (2017), where he writes that: “Just as there is no energy without mass… so there is no time without energy and no energy without time” .